This invention is directed to a volute housing for a centrifugal fan, blower or the like. The theory, design and application of such centrifugal fans can be found in the publications entitled "Turboblowers" by Alexey Joakim Stepanoff, published by John Wiley & Sons, Inc. and available at the Library of the University of Maryland, College Park, Maryland and "Fan Engineering" by Richard D. Madison, published by Buffalo Forge Company, Buffalo, N.Y. (copyright 1949) and also available at the latter noted library. These publications describe several volute housing designs, including a constant velocity volute which is said to be the most favorable for efficiency because of the alleged fact that at the best efficiency point pressure is uniform around the volute. The latter condition is said to be the most desirable for impeller performance. In this design the entire recovery of the kinetic energy into pressure takes place in the volute nozzle which is preferably of a diverging relationship with the included angle being established experimentally at 8.degree. for a circular cone to obtain the most efficient velocity convergent through the nozzle, though a range of 6.degree. to 10.degree. is acceptable. Beyond 10.degree. efficiency is adversely affected. However, in such constant velocity volute housings, the volute pressure is constant until released by the discharge nozzle. The disadvantage of such constant velocity volute housings is that the capacity must be maintained at all times at its rated capacity, otherwise at partial capacities, pressure increases toward larger volute sections and decrease toward smaller volute sections. This decreases efficiency and increases noise.
In an abbreviated volute housing about one-quarter of the impeller periphery discharges directly into the discharge opening without establishing normal volute pressure and velocity distribution prevailing in the remaining three-quarters of the controlled volute housing section. The disadvantage is that the average volute velocity may only be one-half of the absolute velocity at the impeller discharge. Thus sound is decreased but so too is efficiency.
In both the normal volute casing and the abbreviated volute casing, the sidewalls are substantially parallel to each other throughout and it is the peripheral volute wall which progressively diverges from the circular fluid inlet openings in a direction away from the cut-off point or tongue to the volute throat. Essentially, the volute peripheral wall ends at the volute throat and the volute throat defines the initiation or entrance of the discharge nozzle. It is in the area downstream of the throat that the discharge nozzle sidewalls are flared in a direction diverging away from each other in the direction of fluid travel. Such flaring can extend slightly downstream of the volute throat. Such volute casings or housings are generally constructed from galvanized metal and the divergent sidewall angles are extremely abrupt (20.degree.-45.degree.) resulting in excessive turbulence and swirling of discharge fluid/air with an attendant increase in noise.
Another volute housing includes typical circular fluid inlet openings, a volute peripheral wall and sidewalls which continuously diverge from the cut-off point or tongue in the direction of fluid flow to the throat and beyond the discharge nozzle to the discharge opening or orifice. A volute housing so constructed is found in U.S. Pat. No. 3,491,550 in the name of Thomas C. Cavis issued Jan. 27, 1970. This construction increases the RPM's only, and effects expansion from the throat or cut-off point through 360.degree. which basically creates a sound amplification structure typical of the curvature found in a tuba or a french horn. This creates a low bass hum which amplifies the highest sound at the compression point or tongue which is the area of maximum (and virtually only) compression.
From the foregoing, each of the volute housings known suffer from two main disadvantages, namely, (a) low efficiency and (b) high noise.